This invention relates to fiducial mark patterns for graphical bar codes.
A typical bar code symbol is a pattern of parallel bars and spaces of various widths that represent data elements or characters. The bars represent strings of binary ones and the spaces represent strings of binary zeros. A conventional xe2x80x9cone-dimensionalxe2x80x9d bar code symbol contains a series of bars and spaces that vary only in a single dimension. One-dimensional bar code symbols have relatively small information storage capacities. xe2x80x9cTwo-dimensionalxe2x80x9d bar codes have been developed to meet the increasing need for machine-readable symbols that contain more information than one-dimensional bar code symbols. The information storage capacity of two-dimensional bar code symbols is increased relative to one-dimensional bar codes by varying the bar code patterns in two dimensions. Common two-dimensional bar code standards include PDF417, Code 1, and Maxicode. One-dimensional and two-dimensional bar code symbols typically are read by optical scanning techniques (e.g., by mechanically scanned laser beams or by self-scanning charge-coupled devices (CCD""s)) that convert a printed bar code symbol into electrical signals. The electrical signals are digitized and decoded to recover the data encoded in the printed bar code symbol.
Bar codes may be used in a variety of applications, including low information content applications (e.g., automatic price tagging and inventory management), and relatively high information content applications (e.g., encoding mail addresses and postage for automated mail reading and mail distribution systems, and encoding compressed content of a printed page).
As used herein, the term xe2x80x9cgraphical bar codexe2x80x9d broadly refers to an image that contains inconspicuous graphical modulations that encode embedded information. The invention features fiducial mark patterns for graphical bar codes that are characterized by high detection robustness and good local deformation tracking.
In one aspect of the invention, a base image is modulated with a graphical encoding of a message to produce a graphical bar code, and a fiducial mark pattern comprising a plurality of dots arranged to track one or more reference locations and local deformation across the graphical bar code is generated.
Embodiments in accordance with this aspect of the invention may include one or more of the following features.
The fiducial mark pattern may surround the graphical bar code.
The fiducial mark pattern preferably comprises reference color dots and background color dots. The reference color dots and the background color dots may be arranged in a repeating pattern having a characteristic period and comprising one reference color dot and one or more background color dots.
In some embodiments, the reference color dots are arranged along a rectangular fiducial mark path surrounding the graphical bar code. The rectangular fiducial mark path may have a height dimension of 1+Hxc2x7T and a width dimension of 1+Wxc2x7T, where H and W have integer values of 1 or greater and T is the characteristic fiducial mark pattern period. The graphical bar code may have a height dimension of Hxc2x7Txe2x88x922xc2x7xcex94xe2x88x921 and a width dimension of Wxc2x7Txe2x88x922xc2x7Txe2x88x921, where xcex94 is the dot spacing between the graphical bar code and the fiducial mark pattern.
The graphical bar code and the fiducial mark pattern may be rendered with dots of the same size.
In some embodiments, the fiducial mark pattern comprises at least one orientation mark. The orientation mark may be located at a corner position of the fiducial mark pattern. The orientation mark preferably is different from other marks of the fiducial mark pattern.
In another aspect of the invention, fiducial mark candidates are identified in a fiducial mark pattern, a fiducial mark path is computed based upon one or more of the identified fiducial mark candidates, and one or more reference locations are identified based upon the computed fiducial mark path.
Embodiments in accordance with this aspect of the invention may include one or more of the following features.
Global deformations in the graphical bar code may be corrected based upon the one or more reference locations.
Fiducial mark candidates may be identified by identifying centers of reference color dots in the fiducial mark pattern. Identified fiducial mark candidates having dot sizes that are larger than a selected dot size may be discarded. Identified fiducial mark candidates that are spaced from the computed fiducial mark path by more than a selected distance also may be discarded.
A fiducial mark candidate located within a selected range from one of the identified reference locations preferably is identified as a valid fiducial mark and its location preferably is identified as a reference location for a neighboring fiducial mark candidate. A fiducial mark candidate that is spaced outside of a selected range from an identified reference location may be discarded.
In some embodiments, the orientation of the graphical bar code may be identified based upon an asymmetric fiducial mark candidate in the fiducial mark pattern.
Other features and advantages of the invention will become apparent from the following description, including the drawings and the claims.